1. Field of the Invention
The apparatus of the present invention relates to hypodermic needles, and more particularly to a shield for hypodermic needles to protect medical personnel and others from accidental contact with needles that may have been exposed to contagious and/or blood-borne diseases.
2. Art Background
Hypodermic needles are indispensable to the health care industry for both drawing bodily fluids from and administering medication to patients. Most hypodermic needles currently in use are disposed of, after one use, in a waste receptacle to limit further contact between medical personnel or patients and the cannulas of the used needles. Avoiding contact with used needles is essential to prevent the transmission of diseases, particularly blood-borne diseases such as acquired immune deficiency syndrome (AIDS). Although medical personnel are trained to handle used needles with extreme care to avoid exposing themselves, the large volume of unshielded syringes renders such accidental exposure commonplace.
The hypodermic syringes now commonly in use offer insufficient protection against accidental contact with their cannulas after use. The most common hypodermic needle includes a replaceable plastic cap to cover its cannula. The cap is removed just prior to use and is replaced thereafter. The act of replacing the cap exposes medical personnel to the danger of accidental contact, particularly subcutaneous contact, with the used cannula. Moreover, the person administering the needle may forget to replace the cap, or may do so incorrectly, thus increasing the probability of accidental contact between the used cannula and those who must handle medical refuse.
The designs of hypodermic needles now commonly in use also do little to discourage re-use of the needle. In recent years, it has become common for certain persons such as intravenous drug users to illegally use needles discarded by hospitals, clinics and other legitimate users. This illegal use further increases the spread of communicable diseases such as AIDS. The user of discarded needles of current designs must only remove the needle's plastic cap to access the dirty cannula.
A number of prior art needle shields have been developed in an attempt to solve the problems of accidental exposure and illicit re-use. Many of these devices create a "safe" position with a cylindrical outer sleeve that radially surrounds the cannula such that the cannula is not exposed after use. Many of these designs, however, require that the needle administrator take an affirmative act to place the device in the safe position. This is a feature of U.S. Pat. Nos. 5,067,490, 4,935,010, 4,997,422, 5,000,167 and 5,181,524.
These devices suffer from problems similar to those of the common hypodermic needle using a plastic cap. The act of placing the device in the safe position exposes the needle administrator to the danger of accidental contact with the cannula. Moreover, the needle administrator may forget to place the device in the safe position and thus fail to reduce the probability of accidental contact between the cannula and refuse handlers.
Other prior art designs have been developed in an attempt to provide a needle shield that automatically resets to a safe position after the needle is used, thus eliminating the need for an affirmative act by the administrator. These devices also typically employ a sleeve to axially surround the cannula. The cannula is coupled to a spring such that when an axial force of sufficient magnitude is applied by the needle administrator to the syringe, the cannula protrudes from the sleeve to permit injection. The spring develops a reactive axial force in the direction opposite to the applied force. When the administrator releases the axial force as the needle is withdrawn, the spring forces the cannula to retract within the sleeve. This technique is disclosed in U.S. Pat. Nos. 4,917,672, 5,135,510 and 4,915,697. The '697 patent employs a plastic pleating as a spring. The '672 patent discloses a spring fitted around a cannula and a sleeve that is fitted on top of the spring. For this design, the sleeve obscures the cannula and thus makes it more difficult for needle administrators to apply the cannula to a particular point.
The use of a spring to reset the cannula into the safe position suffers from a major drawback. If the spring is chosen with a relatively low spring constant, a very small force, not substantially greater than the force required to cause subcutaneous contact with an unshielded needle, will allow the cannula to protrude from the sleeve. Thus, if a needle administrator accidently provides a small amount of axial force, the cannula can be accidentally exposed. Conversely, if a spring is chosen with a relatively high spring constant, so that significant axial force is required to expose the cannula, the spring will create a resistance that increases as the cannula protrudes further from the sleeve; the reactive force created by a spring increases as the length of the spring is decreased through compression. Thus, to attain an appropriate depth of insertion in the patient, the needle administrator must apply an increasingly greater force than was required to initially disengage the needle from the safe position. This increasing force will be transmitted to the patient, causing discomfort, and will likely render smooth administration of an injection difficult. This inherent property of springs therefore makes it difficult to solve the problems of inadvertent exposure and unacceptably large resistance during the injection process.
Thus, there is much room for improvement in the art of shielding the handles of hypodermic needles from the cannulas of those needles after they have been used.